Technical Field
The present invention relates generally to genetically modified photosynthetic microorganisms, e.g., Cyanobacteria, that overexpress an acyl carrier protein (ACP) and/or an acyl-ACP synthetase (Aas), or a fragment or variant thereof, optionally in combination with one or more additional lipid biosynthesis proteins, to produce high levels of lipids such as fatty acids and/or triglycerides. Also included are related methods of using these genetically modified photosynthetic microorganisms as a feedstock, e.g., for producing biofuels and other specialty chemicals.
Description of the Related Art
Triglycerides are neutral polar molecules consisting of glycerol esterified with three fatty acid molecules. Triglycerides are utilized as carbon and energy storage molecules by most eukaryotic organisms, including plants and algae, and by certain prokaryotic organisms, including certain species of actinomycetes and members of the genus Acinetobacter. 
Triglycerides may also be utilized as a feedstock in the production of biofuels and/or various specialty chemicals. For example, triglycerides may be subject to a transesterification reaction, in which an alcohol reacts with triglyceride oils, such as those contained in vegetable oils, animal fats, recycled greases, to produce biodiesels such as fatty acid alkyl esters. Such reactions also produce glycerin as a by-product, which can be purified for use in the pharmaceutical and cosmetic industries
Certain organisms can be utilized as a source of triglycerides in the production of biofuels. For example, algae naturally produce triglycerides as energy storage molecules, and certain biofuel-related technologies are presently focused on the use of algae as a feedstock for biofuels. Algae are photosynthetic organisms, and the use of triglyceride-producing organisms such as algae provides the ability to produce biodiesel from sunlight, water, CO2, macronutrients, and micronutrients. Algae, however, cannot be readily genetically manipulated, and produce much less oil (i.e., triglycerides) under culture conditions than in the wild.
Like algae, Cyanobacteria obtain energy from photosynthesis, utilizing chlorophyll A and water to reduce CO2. Certain Cyanobacteria can produce metabolites, such as carbohydrates, proteins, and fatty acids, from just sunlight, water, CO2, water, and inorganic salts. Unlike algae, Cyanobacteria can be genetically manipulated. For example, Synechococcus is a genetically manipulable, oligotrophic Cyanobacterium that thrives in low nutrient level conditions, and in the wild accumulates fatty acids in the form of lipid membranes to about 10% by dry weight. Cyanobacteria such as Synechococcus, however, produce no triglyceride energy storage molecules, since Cyanobacteria typically lack the essential enzymes involved in triglyceride synthesis. Instead, Synechococcus in the wild typically accumulates glycogen as its primary carbon storage form.
Clearly, therefore, there is a need in the art for modified photosynthetic microorganisms, including Cyanobacteria, capable of producing lipids such as triglycerides and fatty acids, e.g., to be used as feed stock in the production of biofuels and/or various specialty chemicals.